1. Field of the Invention
The invention relates to a process for the preparation of betaines.
2. Description of the Prior Art
German Auslegeschrift No. 10 62 392 describes germicidal shampoos which are obtained by reacting a fatty acid with a dialkylaminoalkylamine to form an acid amide, which is then quaternized with a halogencarboxylic acid. The betaines obtained by this reaction are surface active and may be used as detergents. They are employed especially in cosmetics, for example, as shampoos and have excellent detersive and foaming capabilities together with a germicidal effect.
More particularly, such betaines have the formula EQU R.sup.1 CONH(CH.sub.2).sub.x N.sup..sym. R.sup.2 R.sup.3 (CH.sub.2).sub.y COO.sup..crclbar. (I)
in which
R.sup.1 is the alkyl residue of a fatty acid with 6 to 18 carbon atoms,
R.sup.2 and R.sup.3 are the same or different and represent alkyl residues with 1 to 4 carbon atoms,
x is 2 or 3, and
y is 1, 2 or 3.
Betaines having formula I are prepared by quaternizing the fatty acid amide of the formula EQU R.sup.1 CONH(CH.sub.2).sub.x NR.sup.2 R.sup.3 (II)
with .omega.-halogenalkylcarboxylic acids of the formula EQU X(CH.sub.2).sub.2 COOH (III)
in which X is a halogen residue or salts of such acids.
In German Pat. No. 11 72 802, a bath salt is described which is characterized by the fact that it contains selected betaines. If this bath salt is added to bath water and the jet of water running into the bath is directed onto this bath salt, a dense, cream-like foam is formed which is stable even in the presence of soap.
These betaines are used on a large scale for the aforementioned purposes. At the same time, it turns out occasionally that persons with a very sensitive skin develop skin irritations which are evidently caused by impurities unavoidably contained in the betaines as a result of the manufacturing process.
Extensive investigations have led to the conclusion that the residue of fatty acid amide dialkylamine which has not been quaternized and the organically linked chlorine are responsible for the skin irritations. Commercially available betaines of the aforementioned formula contain from about 0.3 to 3 weight percent of free fatty acid amides.
As is well known, the quaternization reaction with halogencarboxylic acid is not quantitative but proceeds up to about 90% of the theoretical conversion, even with an excess of halogencarboxylic acid.
Many attempts to remove the unquaternized product from the reaction mixture or to convert it into a compound which does not irritate the skin have been made. For example, a process is described in German Offenlegungsschrift No. 20 63 422 which converts unreacted fatty acid amide dialkylamine into the corresponding amine oxide by the addition of hydrogen peroxide. A mixture is then obtained which consists of amine oxides as well as the desired betaine. Because of the use of larger amounts of perhydrol required, this procedure is expensive and requires an additional processing step. A further disadvantage of the conversion of the amineamides into the amineoxide amides lies in the fact that the reaction also does not proceed quantitatively. In addition, it is necessary to make certain that any excess peroxide is destroyed in order to prevent peroxide-induced skin irritations.